the behaviour of honeybees visiting flowers of fruit trees

The Behaviour of Honeybees Visiting Flowers of Fruit TreesAuthor(s): J. B. FreeSource: Journal of Animal Ecology, Vol. 29, No. 2 (Nov., 1960), pp. 385-395Published by: British Ecological SocietyStable URL: http://www.jstor.org/stable/2211 .

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385

THE BEHAVIOUR OF HONEYBEES VISITING FLOWERS OF FRUIT TREES

BY J. B. FREE

Rothamsted Experimental Station*

This investigation is concerned with some important aspects of the foraging behaviour of bees on fruit trees about which information was scanty or contradictory.

Bees foraging on fruit trees were chosen at random and continuously observed for as long as possible. Records were made of: whether they collected nectar, or pollen, or both; the way they worked the flowers; the numbers of flowers and trees visited; the length of time they were observed; the reason for observation ending. Table 1 describes the orchards and fruit varieties used. A few observations were also made on other varieties, but the behaviour of bees was similar to that given below.

RESULTS

Bees foraging on apricot, peach, pear, plum, or sweet cherry made the following types of flower visits: (a) scrabbled over the anthers to obtain pollen (as described by Parker 1926), and did not attempt to collect nectar; (b) stood on the stamens or petals and pushed their tongues and front part of their bodies among the stamens to reach the nectary, invariably touching some of the anthers because of the spreading arrangement of the stamens; (c) behaved as in b but scrabbled over the anthers for pollen afterwards; or (d) behaved as in b but scrabbled over the anthers for pollen first. Most bees made either a visits only or b visits only during observation, but 13 7 0/ of them behaved in two or more of the above ways (Table 2). Nearly all these that behaved in more than one way collected pollen only on some flower visits, and made a mean of 8-3 a visits and 7-4 b, c or d visits.

When observation began on a bee the presence or absence of pollen in its corbiculae was noted (Table 3). Except on 6 May (apricots), when pollen was scarce, most bees foraging for pollen only (a visits) carried pollen. But bees which were constant to b visits often did not, and the proportion of them with pollen varied greatly from day to day; probably such bees were mainly nectar-gatherers but collected pollen incidently when it was obtained readily. Bees making b visits packed pollen into their corbiculae only during flight, whereas those making a visits often did so on the flowers.

In contrast to the spreading stamens of other species, those of many apple varieties are upright and bees can obtain nectar from them by pushing their tongues between the filaments without touching the anthers or stigma (type e visit). Observations were made on five varieties, which differed in the length and

* Work done while on secondment to Ontario Agricultural College.



0o

Table 1. Orchards in which observations were made

Distance (ft) between Date of Mean Mean adjacent trees

Species Location observation temperature Varieties observed diameter - (0 F) during of trees In same In different observation (ft) row row

Peach Sandhill's Research Station, 14 March 62-3 Hiland 12-0 15 * (Prunus persica) N. Carolina

16 ,, 52-2 20 ,, 65-0 Elberta 12 9 ,, 20

(approx.) Z

Apricot Nr. Niagara-on-the lake, 1 May 52'0 Delicious and Stellas 14 5 20 20 (Prunus armenica) Ontario (Delicious) 0

3,, 55.0 ,, 189 ,,

(Stellas) ,, ~~~~4 ,, 63-9 ,

Horticultural Exp. Sta., 6 ,, 51-9 Numerous 17-7 25 25 o Vineland, Ontario

Sweet cherry Nr. Queenstown, Ontario 5 May 65-8 Black Tartarian and Early 16*0 20 25 ; (Prunus avium) Lyons

7 ,, 650 Hedelfingen and Schmidt 22'5 25 25 Plum Nr. Vineland, Ontario 13 May 58-0 Grand Duke and Reine Claude 18-0 20 20

(Prunus domestica) ,, 17 ,, 49-1 ,,

Pear Nr. Vineland, Ontario 17 May 51 7 Bartlett and Kieffer 13 9 16 20 (Pyrus communis) ,, 19 ,, 516 ,. .. ,I ..

Apple Horticultural Exp. Station, 22 May 70 6 Red Gravenstein and exp. 14 4 18 19 (Pyrus malus) Vineland, Ontario variety 25051

24 ,, 60 5 Bramley, Oriole and Sandow 14 5 18 19

* Only one row in flower.



J. B. FREE 387

thickness of their filaments (Table 4). Experimental variety 25051 and Red Gravenstein were observed on 22 May and the others on 24 May, the variety observed being changed about every 30 min. Varieties with thin filaments (Sandow and 25051) had more b than e visits but varieties with filaments of

Table 2. Number of bees which made different types offlower visits during observation (for explanation see text)

Peach Apricot Sweet cherry Plum Pear Total

a only 148 140 30 54 47 419 b only 122 63 64 56 13 318 c only 2 1 12 0 1 16 d only 1 2 2 0 0 5 a and b 29 12 11 4 2 58 a and c 3 1 2 0 0 6 a and d 1 0 0 0 0 1 b and c 3 1 10 1 4 19 candd 0 0 0 0 1 1 a, b and c 5 5 7 4 5 26 a, b and d 1 2 0 0 0 3 a, c and d 1 0 0 0 0 1 b, c and d 0 0 0 0 2 2 a,b,candd 2 0 1 0 0 3

medium thickness (Red Gravenstein and Oriole) had more e than b visits, probably because the greater rigidity and height of the filaments made it more difficult for bees to force their way from the top of the stamens to the nectaries. During many e visits to Sandow and 25051 the bees pushed their heads and front part of

Table 3. Presence of Pollen in corbiculae and type of visit

Bees constant to a visits Bees constant to b visits

Pollen Pollen Pollen Pollen present absent present absent

r14 March 69 6 13 21 Peach 16 ,, 45 7 2 3

L20 ,, 20 0 63 13 f 4 May 43 1 23 16

Apricot l 6 ,, 33 12 2 16

5 May 9 1 9 22 Sweet cherry 7 20 1 19 9

cr 13 ,, 14 0 1 1

Plum <( 17 14 0 1 0 Plum ~ I~19 12 0 1 43

Pear 18 May 27 3 7 5

their bodies among the filaments and often touched the anthers, but during e visits to Red Gravenstein and Oriole they could only push their tongues through the more closely spaced and taller filaments and rarely touched the anthers. The thick, closely set filaments of Bramley flowers prevented bees inserting their tongues between them, and although bees probably had more difficulty in making



388 Behaviour of honeybees

b visits on this variety than on the two preceding ones, the proportions of e visits was lower than with the thin filament varieties. The difficulty bees had in obtain- ing nectar from the different varieties is reflected in the mean number of flower bees visited per tree: Sandow, 11-5; 25051, 97; Red Gravenstein, 83; Oriole, 8-4; Bramley, 5-7.

With all species, the percentage of flower visits for pollen only often varied greatly on different days and even at different times on the same day (Table 5). Bees appear to collect pollen only when there is no nectar or it is relatively unattractive; on some days, usually with low temperature, all, or nearly all, visits were for pollen only (16 March, 1, 3, 6, 17 May). On the other days the percentage of visits.for pollen only either decreased (4, 5, 18, 19 May) or increased (20 March, 7 May) during observation, or reached peaks at beginning and end of the day (14 March, 13 May).

As the bees were unmarked, there is no proof that the same bees were involved in a general change over from pollen to nectar collection or vice versa during the day. However, this seems likely because some bees visited some flowers for pollen

Table 4. Behaviouy of bees on apple flowers

Number bees* which pre-

Varieties Length of Width of Number of flower dominantly made stamens (mm) filaments visits of types: visits of types:

Mean Max. a b c d e a b c

Sandow 7.5 8-5 Thin 152 428 22 0 87 3 16 2 Exp. 25051 7.5 9-0 Thin 24 287 12 1 104 1 12 4 Red Gravenstein 10.5 12-0 Medium 27 72 0 0 345 1 2 15 Oriole 12-5 14-0 Medium 85 32 1 3 146 4 1 7 Bramley 13-5 14-5 Thick 98 99 12 1 17 3 5 0

* Visiting 10 or more flowers.

only and others for nectar during the same trip, occasionally changing over from one consistent type of behaviour to another, during observation. Furthermore, on many occasions pollen was still plentiful when most bees were collecting nectar. Exceptions to this condition were in the sweet cherry orchard on 5 May and the plum orchard on 19 May; by the end of observation on these 2 days about half the flowers were denuded of petals, few had any pollen left and nearly all flower visits were for nectar only. In the apricot orchard on 6 May both nectar and pollen were scarce. Bees often approached to within a few millimetres of the flowers without alighting, and appeared able to detect whether or not the flowers had nectar or pollen.

Bees that were constant to one type of visit during observation spent less time per a visit than b visit for every species except pear (Table 6). No correlation was found between the time of day and duration of visit, possibly because the number of observations were too few to show this.

Observations on a bee ceased either because its identity was no longer certain, it flew off the tree concerned and either disappeared or flew to another tree and



J. B. FREE 389

could not be identified, or it went home. A bee going home often first visited one or two flowers on a different part of the tree from that on which it had been working, then cleaned itself (mean duration 27-3 sec: 40 observations) while clinging to a petal or leaf before taking flight, and frequently orientated before finally departing.

Because the bees observed were selected at random, it is likely that, on average,

Table 5. Percentage of flower visits for pollen only (total number of flower visits in parentheses)

Hourly periods of observation beginning

09 00 10.00 11 00 12-00 13-00 14-00 15-00 16-00 17-00 18-00

Peach 14 Mar. 97 88 15 56 85 87 96 - - -

(155) (300) (127) (94) (150) (172) (103) 16,, - 92 96 81 87 90 - - - -

(83) (139) (98) (201) (48) 20,, - 15 27 24 26 44 41 87 - -

(370) (100) (191) (130) (244) (145) (115)

Apricot 1 May - - - 100 100 - - -

(243) (120) 3,, - - 100 100 - - - - - -

(178) (82) 4,, - 85 53 82 70 56 44 54 0 -

(254) (86) (226) (183) (323) (267) (377) (3) 6,, - 88 85 73 87 78 - - - -

(428) (313) (291) (337) (488)

Sweet cherry 5 May - 28 48 14 5 5 1

(76) (474) (147) (411) (259) (486)

7,, - - 24 46 34 57 65 - - -

(107) (110) (250) (317) (221)

Plum 13 May - 100 83 - - - 42 56 84 (70) (160) (24) (45) (219)

17,, - 98 99 100 100 - - - - - (203) (76) (14) (4)

19,, - - - 100 - 89 69 44 24 0

(22) (343) (261) (161) (152) (112) Pear 17 May - - - - - - 100 100 - -

(83) (59) 18,, 100 44 90 46 42 8 19 3 27 -

(38) (139) (118) (171) (45) (206) (220) (108) (127)

they were near the middle of their visit to the tree concerned and altogether visited twice as many flowers before leaving it as they were seen to visit. The number of flowers bees visited per tree was estimated on this basis (Table 7). Information on bees whose identity became uncertain before they left a tree is not included in the calculation. Similarly, doubling the mean number of flowers seen to be visited, on one or more trees, by bees which were followed until they flew home gives an estimate of the mean number of flowers bees visited during a

M J.A.E.




complete trip. Using this information to calculate the number of trees visited per trip (Table 7) gives, if anything, an under-estimate because, owing to the difficulty of following a bee for numerous flower visits and especially from tree to tree, the bees which were observed until they flew home probably tended to be nearer the end rather than the beginning of their trips.

Two pollen-gatherers and two nectar-gatherers were observed for what were most probably complete trips; the pollen-gatherers made 89 (apricot) and 38 (pear) a visits respectively and the nectar-gatherers made 76 (pear) and 82 (sweet cherry) b visits.

Observations on peach flowers did not record when bees returned home, and so the number of trees visited per trip could not be calculated. However, the peach

Table 6. Mean time (sec) per flower visit (number of flowers in parentheses)

Peach Apricot

a visits b visits a visits b visits

14 March 14-4 (681) 32-3 (166) 1 May 10 9 (358) 16 ,, 15.9 (350) 24.4 (12) 3 ,, 8-8 (197) - 20 ,, 15-7 (251) 19-9 (671) 4 ,, 7-3 (1018) 12-2 (395)

6 ,, 6-8 (1447) 9 0 (292)

Total 15-0 (1288) 22-4 (849) Total 7-6 (3020) 10-8 (687)

Sweet cherry Pear


5 May 9 3 (268) 9-5 (789) 17 May 10 0 (109) -

7 ,, 8-7 (270) 11*0 (436) 18 ,, 9 1 (320) 7-8 (435)

Total 9 0 (538) 10 0 (1225) Total 9-3 (429) 8-8 (435)

Plum Apple

a visits b visits a visits b visits e visits

13 May 5-8 (380) 16-5 (74) 22 May 6-2 (4) 8-9 (107) 11 9 (104) 17 ,, 7-3 (247) - 24 5-5 (221) 7-5 (273) 10-4 (91) 19 ,, 4-8 (523) 16-0 (351)

Total 5-6 (1150) 15-6 (436) Total 5-5 (235) 7-9 (380) 11-2 (195)

trees were small with few flowers so it was not so difficult as with other species to follow bees from tree to tree. Bees foraging on peach flowers evidently behaved very differently from those on other species, because the mean number of trees visited during observation and the mean number of flowers visited per tree was 2-3 and 4-2 respectively for 147 bees which made a visits only, and 2-3 and 3-2 for 123 bees which made b visits only. The numbers of trees visited increased with the numbers of flowers visited (Table 8). Even if the mean number of flowers visited was as low as half the maximum observed, the bees would have visited as many as four or five trees a trip.

The mean number of flowers visited per tree varied from day to day (Table 9). In general, with the same varieties, on relatively favourable days when there were



J. B. FREE 391

Table 7. Number of flowers visited per tree and number of trees visited per trip

Mean number Mean number of flowers (y) of flowers (x) Probable visited by Estimated Estimated

visited mean number bees which mean number mean number per tree of flowers probably of flowers of trees during visited returned home visited visited

observation per tree at finish of per trip per trip (number (2x) observation (2y) (y/x)

of trees in (number parentheses) of bees in

parentheses)

Apricot (Niagara-on-the-lake) Type a visits 16-8 (78) 33-6 25-8 (9) 51-6 1-5 Type b visits 8-4 (56) 16-8 17-7 (3) 35-4 2-1

Apricot (Vineland) Type a visits 26-8 (40) 53-6 46-3 (13) 92-6 1P7 Type b visits 12-6 (19) 25-2 13-6 (7) 27-2 1

Sweet cherry Type a visits 17-5 (26) 35 0 26-9 (8) 53-8 i-5 Type b visits 13-7 (88) 27-4 37-4 (12) 74-8 2.7

Plum Type a visits 17-4 (56) 34-8 28-3 (14) 56-6 i 6 Type b visits 5-4 (71) 10-8 7-3 (14) 14-6 1P3

Pear Type a visits 9-2 (56) 18-5 15-3 (17) 30-6 1P6 Type b visits 21-7 (16) 43-4 43-8 (6) 87-6 2-0

Apple All types of visit 9-8 (85) 19-6 22-1 (11) 44-2 2-2

a considerable number of b visits (i.e. peaches, 14 March; apricots, 4 May; plums, 19 May; pears, 18 May; see Table 1) the bees visited more flowers per tree and spent less time per flower than on less favourable days. Records were too few to establish any relationship between the time of day and the number of flowers visited per tree, especially because some bees were obviously satisfied with the crop when others were not. But, in unfavourable weather, and particularly just

Table 8. Number peach flowers and trees visited Per observation

Type a visits Type b visits

Number of Number Mean Number Mean flowers of bees number of of bees number of visited observed trees visited observed trees visited

1- 5 69 1*2 70 1-3 6-10 29 2-0 25 2.9

11-15 19 3-3 14 3-6 16-20 11 3-4 7 5*0 21-25 9 4-3 3 5 0 26-30 6 4-8 0 - 31-35 0 - 2 5-0 36-40 0 - 0 - 41-45 2 7 0 1 7-0 46-50 1 7*0 0 - 51-55 1 8-0 1 7-0




before rain, bees frequently flew from tree to tree, often without even alighting on the flowers. However, on favourable days when many bees were foraging, they appeared to disturb each other more, so that many of them took flight, sometimes going to another tree. In some orchards the distance between rows of trees was

Table 9. Mean nutmber of flowers visited per tree on different days (number of trees in parentheses)

Peach Apricot n ~ ~~~~ A


14 March 4.6 (170) 2-8 (66) 1 May 12-1 (30) 16 ,, 4-1 (99) 1-8 (12) 3 1, l3-7 (19) - 20 3-5 (73) 3-4 (209) 4 ,, 17.8 (59) 8-4 (56)

6 ,, 26-8 (40) 12 6 (19)

Sweet cherry Pear


5 May 28-7 (6) 17-9 (44) 17 May 6-8 (21) -

7 14-5 (19) 8-2 (38) 18 10-7 (35) 21-7 (16)

Plum Apple

a visits b visits All types of visit

13 May 1681 (20) 6-1 (73) 22 May 9 0 (42) 17 9-5 (19) I 0 (1) 24 7-4 (28) 19 ,, 2636 (16) 5*4 (58)

greater than between trees in the same row, and bees flew more frequently between trees in the same row than between trees in different rows (Table 10).

Sweet cherry trees had more bees on their sunny sides than on their shady sides, although the latter were more protected from a strong wind prevailing at the time

Table 10. Effect of greater distance between rows than between trees in a row

Mean distance (ft) Distance between Mean betwveen edges Number of bees

trees (ft) diameter of trees in: changed to trees in: -)of trees (ft) {-&-+i

Adjacent Adjacent Adjacent Same row row Same row row Same row row

Pear 16 20 13-9 2-1 6-1 34 5 Peach 15 20 12-9 2-1 7-1 146 72 Sweet cherry 18 25 16.0 2-0 7-0 27 11

(Table 11). This difference was also found with Stellas apricot trees but not with Delicious apricot trees which were every third tree in the same rows as the Stellas. The Delicious trees were smaller than the Stellas (mean diameter of 14-5 and 18-9 ft respectively), much less dense and with fewer flowers, and more light passed through them.



J. B. FREE 393

DISCUSSION AND CONCLUSIONS

Parker (1926) observed that bees visiting apple, cherry, plum or pear flowers collected either nectar only, pollen only, or both. However Vansell (1942) working with cherry and peach, and Stephen (1958) with pear, found that nectar-gatherers became covered with pollen, and Brown (1951) found that bees visiting plum flowers collected either nectar or pollen, but never both during the same trip. My results agree with those of Parker (1926), and although all bees collecting nectar from every species except apple touched the anthers, only about half collected pollen as well, probably doing so incidently when the pollen was readily available. A few bees deliberately collected pollen as well as nectar on the same visit, scrabbling over the anthers in the same way as bees collecting pollen only.

The upright stamens of apple flowers allow bees to obtain nectar without touching the anthers (Brittain 1933, Free 1958). My results show that the proportion of nectar-gatherers which do so depends on the structure of the stamens, and if the stamens are flexible enough, bees prefer to approach the nectary from the top of the stamens, as in the other species studied. Preston (1949) believed that

Table 11. Mean number of bees on sunny and shady side of trees (nutmber of trees in parentheses)

Sunny sides Shady sides

Apricots Row 1 Delicious 0-6 (15) 0-5 (15) Stellas 7-9 (9) 2-9 (9)

Row 2 Delicious 0-8 (15) 0 9 (15) Stellas 10-8 (10) 5-1 (10)

Row 3 Delicious 1-8 (19) 1-6 (19) Stellas 17-9 (10) 8-1 (10)

Sweet cherry Row 1 1-6 (20) 1 0 (20) Row 2 2-9 (20) 0-6 (20)

the tall, thick stamens of Bramley apple flowers prevented honeybees from reaching the nectaries; this has been confirmed. Probably the behaviour differences observed on different apple varieties are more consistent in orchards with only one or two varieties present, than in the experimental orchard which contained numerous varieties.

Although individual bees tended to be constant to one type of flower visit during a single trip, certain bees visited some flowers for pollen only and others for nectar during the time they were observed. It is unlikely that these bees were primarily pollen-gatherers which were taking only enough nectar to sustain themselves, because they made similar numbers of flower visits for pollen only and for nectar.

The ratio of nectar-gatherers to pollen-gatherers, and probably the behaviour of individuals, varied greatly on different days and at different times on the same day, and bees apparently can often collect pollen when nectar is either unavailable or unattractive. These results agree with those of Brittain (1933) on apple.

A bee does not forage at random over a crop but tends to restrict itself to a




small part of it (Minderhoud 1931, Buzzard 1936, Butler, Jeffree & Kalmus 1943), and Minderhoud (1931) supposed that large fruit trees, well separated from others, might not be sufficiently cross-pollinated. MacDaniels (1931) said that a bee tends to keep to one tree, or two adjacent trees, and Singh (1950) found that 45 out of 66 bees kept to one apple tree during observation. Because of the difficulty in maintaining continuous observation on a bee, and especially in following it from tree to tree, I could not obtain definite information on the number of trees a bee visits during a single trip, but relevant calculations indicate that, on the average, bees probably visit two or more.

During relatively unfavourable conditions, bees tended to spend more time per flower and visited fewer per tree than in favourable conditions. The observed extension of foraging areas in unfavourable circumstances agrees with observations on the behaviour of bees on artificial food sources (Butler et al. 1943) and indicates that, although fewer bees may be foraging in such circumstances, they are individually more valuable as cross-pollinators. Bees collecting nectar change trees more frequently than those collecting pollen only, but as they also spend more time per flower visit, and are probably less efficient pollinators, particularly of apple, they are probably less valuable as cross-pollinators.

SUMMARY

1. Foraging bees were watched on apple, apricot, peach, pear, plum and sweet cherry flowers.

2. During a single flower visit a bee either approached the nectary from the top of the stamens, sometimes collecting pollen in the process, or scrabbled over the anthers and gathered pollen only, or, far less often, did both. Bees also obtained nectar from apple flowers by inserting their tongues between the filaments of the stamens from the side, often without touching the anthers; the proportion of nectar-gatherers that visited the flowers in this way varied with the flexibility, height and thickness of the filaments of the variety concerned.

3. During a single trip or part of a trip most bees kept to one type of behaviour, but some collected pollen only on some flower visits, and nectar, with or without pollen, on others.

4. The proportions of the different types of flower visits varied from day to day and at different times on the same day. Pollen was sometimes collected when little or no nectar was collected, and on two occasions when the flowers concerned were nearly denuded of pollen, the majority of bees collected nectar only.

5. Except on pears, pollen-gatherers spent less time per flower and visited more flowers per tree than nectar-gatherers. On unfavourable days bees spent longer per flower and visited fewer flowers per tree.

6. Bees probably visited two or more trees per trip and tended to change to trees nearest to those on which they started working.

7. More bees foraged on the sunny than on the shady sides of trees.



J. B. FREE 395

ACKNOWLEDGMENTS

It is a pleasure to thank Mr S. C. Jay of Manitoba University for helping with the observations on peach flowers, and the following for their ready co-operation: Prof. G. F. Townsend of Ontario Agricultural College, Messrs C. Black and W. A. Stephens of North Carolina State College, Mr G. H. Dickson of the Horticultural Experiment Station and Products Laboratory, Ontario. My colleagues at Rotham- sted Experimental Station kindly criticized the manuscript. This work was done during the tenure of a fellowship from the National Research Council of Canada.

REFERENCES Brittain, W. H. (1933). Apple pollination studies in the Annapolis Valley. Bull. Dept.

Agric. Can. N.S. No. 162, 1-198. Brown, A. G. (1951). Factors affecting fruit production in plums. Fruit Yearb. 1950, 12-18. Butler, C. G., Jeffree, E. P. & Kalmus, H. (1943). The behaviour of a population of

honey-bees on an artificial and on a natural crop. J. Exp. Biol. 20, 65-73 Buzzard, C. N. (1936). De l'organisation du travail chez les abeilles. Bull. Soc. Apic.

Alpes-Marit, 15, 65-70. Free, J. B. (1958). Attempts to condition bees to visit selected crops. Bee World, 39, 221-30. MacDaniels, L. H. (1931). Further experience with the pollination problem. Proc. N.Y.

St. Hort. Soc. 76, 32-7. Minderhoud, A. (1931). Untersuchungen ueber das Betragen der Honigbiene als Bliiten-

bestauberin. Gartenbauwiss, 4, 342-62. Parker, R. L. (1926). The collection and utilization of pollen by the honeybee. Mem. Cornell

Agric. Exp. Sta. No. 98, 1-55. Preston, A. P. (1949). An observation on apple blossom morphology in relation to visits

from honeybees (Apis mellifera). Rep. E. Malling Res. Sta. 1948, 64-7. Singh, S. (1950). Behaviour studies of honeybees in gathering nectar and pollen. Memoir

Cornell Univ. Agric. Exp. Sta. No. 288, 1-57. Stephen, W. P. (1958). Pear pollination studies in Oregon. Ore. St. Coll. Tech. Bull. 43,

1-43. Vansell, G. H. (1942). Factors affecting the usefulness of honeybees in pollination. U.S.

Dept. Agric. Circ. 650, 1-31.



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